A disk refiner comprises two opposed counter-rotating refining disks, one or both of which are rotary within a surrounding refiner housing. A plurality of refining elements is arranged on the refining disks. A refining element can also be assembled from several partial elements. These refining elements are formed with a pattern of bars and intermediate grooves. The refining disks are located in such a way that the refining elements form a refining gap, through which, the fiber material is intended to pass from the inside out, and during that passage the disintegration is carried out by the bars of the elements. These bars can be of different designs. They can be continuous or discontinuous, with uniform or varying height. In certain cases toothed bars can be used.
In the refining gap between the refining surfaces the fiber material is first defibered, i.e. the fibers are separated. This takes place in the interior portion of the refining gap, where there is the greatest distance between the refining surfaces. The refining gap decreases thereafter outwardly, and at the same time the pattern of the bars becomes tighter in order to obtain the desired working of the fiber material. Large amounts of energy are required to achieve this working. The material concentration can be from about 3% to 50%, which generates large amounts of steam from the water following therealong.
Part of the generated steam flows rearwardly inward to the inlet for the material, and another portion flows forwardly outward to the outlet from the refining gap. The steam pressure increases from the inlet to a pressure maximum in the outer portion of the refining gap, and thereafter decreases toward the outlet. This maximum pressure, which can rise to from about 7 to 8 bar, causes the steam generated in the refining gap outside the pressure maximum to flow outward, while the steam inside the pressure maximum flows inward. The rearward flowing steam causes a disturbance in the fiber flow in the refining gap. This is inconvenient, because a non-uniform fiber flow results in uneven pulp quality.
Depending on the desired degree of working and, thus, pulp quality, the refining surfaces are given a different design. Other factors also influence the pulp quality, for example the size of the refining gap, the moisture content of the fiber material, the feed, the temperature, and other such factors.